1. Field of the Invention
This invention relates to a method for reducing an inferior bonding at a joint surface in a so-called hydrogen ion delamination (separation) method (also called as a smart-cut method) in which joined SOI wafers, at least one wafer being ion implanted, are heat treated to cause separation to SOI wafers and delaminated wafers, thereafter the SOI wafers are heat treated to make bonding firmly.
2. Description of the Related Art
As a method of manufacturing an SOI (silicon on insulator) wafer under application of a bonding method, it has been well known in the related art to provide a technology for bonding two silicon wafers through a silicon oxide film, for example, a method for forming an oxide film on at least one wafer, closely contacting these wafers to each other without placing any foreign materials at the joint surface, thereafter heat treating them at a temperature of about 200 to 1200.degree. C. to increase a bonding strength, as disclosed in the gazette of Japanese Patent Publication No. Hei 5-46086.
The bonded wafer of which bonding strength is increased through performing a heat treatment is enabled to be ground and polished after the treatment, so that the wafer applied to form an element is reduced in its thickness to a desired thickness under grinding and polishing operations and then it is possible to form the SOI layer forming an element.
The bonded SOI wafer made in this way has some advantages that it has a superior crystalline characteristic of the SOI layer and a high reliability in an buried oxide layer present just below the SOI layer. However, it was made thin through grinding and polishing, resulting in that it took much time for making a thin film, useless application of material was set and uniformity of film thickness of only about .+-.0.3 .mu.m of a target film thickness was attained.
In turn, with a recent trend of high integration and high speed in operation of semiconductor devices, it is required to have further reduction in the thickness of the SOI layer and a uniformity of film thickness and more practically, it is required to have a film thickness and its uniformity of about 0.1.+-.0.01 .mu.m.
Realization that the thin film SOI wafers having such a film thickness as above and a uniform film thickness by a bonded wafer was impossible under a thickness reducing work through the related art grinding and polishing, so that as a new thin film forming technology, a method called as a hydrogen ion delamination (separation) method (also called as a smart cut method) disclosed in the gazette of Japanese Patent Laid-Open No. Hei 5-211128 has been developed.
This hydrogen ion separation method is a technology on which an oxide film is formed on at least one of the two silicon wafers, either hydrogen ion or rare gas ion is implanted from the upper surface of one silicon wafer, a fine air bubble layer (enclosing layer) is formed in the silicon wafer, thereafter the ion implanted surface is closely contacted with the other wafer through the oxide film, a heat treatment (a separation heat treatment) is applied, one wafer is delaminated in a thin film state with the fine air bubble layer being applied as an separation surface (a delaminating surface), and further a heat treatment (a bonding heat treatment) is applied and they are forcedly bonded to each other to make an SOI wafer.
This method has an advantage that the material can be used effectively due to the fact that the delaminating surface is a superior mirror surface and the SOI wafer having a quite high uniform characteristic of the SOI layer can be attained in a relative easy manner and further the delaminated one wafer can be utilized again.
In addition, this method also enables the silicon wafers to be directly bonded to each other without applying any oxide film, and this is used not only for a case in which the silicon wafers are bonded to each other, but also a case in which insulator wafers having different thermal expansion coefficients such as quartz, silicon carbide and alumina or the like after implanting ions into the silicon wafers.
In the case that the bonded wafers are made by such a hydrogen ion separation method, it is also possible to perform a heat treatment of high temperature also acting as a separation heat treatment and a bonding heat treatment at once as a post heat treatment after the two wafers are closely contacted to each other at a room temperature, although it is normally carried out that for a sake of convenience in use of the delaminated wafer after its heat treatment, the separation heat treatment is once carried out at a relative low temperature of about 500.degree. C., the delaminated wafer is recovered, only the SOI wafer is separately processed in a bonding heat treatment at a high temperature of about 1100.degree. C., for example.
However, the present inventors found that the following two kinds of inferior bonding occurred frequently while the bonded wafer was being made in repetition by the hydrogen ion separation method.
1) Inferior bonding in which after performing the separation heat treatment, delaminated state does not occur at the ion implanted layer, but either an entire surface or its part is separated at the close contacted portions (joint surface):
2) Inferior bonding in which although a void (a partial unbonded portion) is not generated at all after performing a separation heat treatment, but void is generated after performing a bonding heat treatment.
In regard to the aforesaid two kinds of inferior bondings, the present inventors studied them honestly and found that these inferior bondings had a certain relation with a concentration of organic substances contained in the surfaces closely contacted to each other at a room temperature.
Further, describing it in detail, it is assumed that the phenomenon in which the organic substances may become a cause of void is generated in the case that the organic substances trapped at the joint surface are processed by a high temperature bonding heat treatment to become gasified, and if its gas pressure is superior to a bonding strength or rigidity of silicon, void is formed. Accordingly, in the case of the related art bonding method, since it is normally found that the bonded wafer is not made thin when a bonding heat treatment is carried out, if the wafer has a high rigidity and a relative small amount of organic substances is trapped at the joint surfaces, they may not become gasified and formed as void and they are diffused and diminished from the joint surface.
To the contrary, in the case of hydrogen ion delamination method, since a high temperature bonding heat treatment is carried out at the SOI layer after the separation heat treatment under a state of quite thin thickness of 2 .mu.m or less, the present inventors have found that rigidity of wafer is quite weak and even if such a small amount of organic substances trapped at the joint surface as one not producing any void under the related art bonding method is present, there occurs a phenomenon in which the void is generated only after the bonding heat treatment is carried out, as described in the above item 2).
In turn, the present inventors have found that occurrence of inferior bonding is dependent on a presence or a non-presence of a silicon oxide film or its thickness at the close contacted surfaces of the wafers. That is, if the bonding is performed through an oxide film, the gas generated under the presence of the organic substances is absorbed into the oxide film, so that the void is hardly formed. Then, it may be considered that this phenomenon shows a certain effect as a thickness of the oxide film is thicker.